Pink ward the bush next to red buff or have Oracles and you're good to go.

Having Baron warded doesn't mean they'll see it because they will have no vision of anything happening on the minimap. They would have to hover over Baron to see that something is attacking it to be able to see that this is happening.

If they don't move their camera on Baron, they will have no knowledge of this.

Yeah I don't think people realize how this works. Nobody is going to check baron when they have vision on it and you don't show up on the minimap (because you are outside the pit). Especially if the rest of your team is visible in lanes.